Method for manufacturing steel for high-strength hollow spring

ABSTRACT

A method for manufacturing steel, by quenching and tempering a seamless pipe for use as a material of a hollow spring, where the seamless pipe including predetermined components is subjected to a heat treatment which is performed to satisfy quenching conditions (1) and tempering conditions (2),
         (1) quenching conditions:
 
26,000≤( T 1+273)×(log( t 1)+20)≤29,000
 
900° C.≤ T 1≤1,050° C.,
 
10 seconds≤ t 1≤1,800 seconds,  formula (1)
   where T1 is a quenching temperature (° C.), and t1 is a holding time (seconds) in a temperature range of 900° C. or higher, and   (2) tempering conditions:
 
13,000≤( T 2+273)×(log( t 2)+20)≤15,500
 
 T 2≤550° C., and
 
 t 2≤3,600 seconds,  formula (2)
   where T2 is a tempering temperature (° C.), and t2 is a total time (seconds) from start of heating to completion of cooling.

TECHNICAL FIELD

The present invention relates to a method for manufacturing steel for ahigh-strength hollow spring. The term “steel for a hollow spring” asused in the present specification means steel obtained by quenching andtempering a seamless pipe for use as a material for a hollow spring.

BACKGROUND ART

With increasing demands for reducing the weight or enhancing the outputof automobiles or the like, springs, such as valve springs, clutchsprings, and suspension springs, which are used in the engine, clutch,suspension, etc., tend to be higher strength and thinner diameters.Together with this, the properties required for springs, including theresistance to hydrogen embrittlement, the fatigue resistance, and thesetting resistance, are becoming increasingly higher. It is stronglydesired to provide a spring steel that can manufacture a springexcellent in these properties.

To produce lightweight springs that are excellent in the springproperties, such as the resistance to hydrogen embrittlement and thefatigue resistance, pipe-shaped hollow steels with no weld bead, i.e.,seamless pipes are used as material for a spring steel, in place ofsolid steels, such as a steel bar, which have been used before. Theseamless pipe is also called a seamless steel tube.

However, when using the seamless pipe as the material for hollowsprings, various problems occur, especially, in terms of manufacturingseamless pipes. That is, to ensure the fatigue strength of the solidsteel for use as the material for springs, which are not hollow,generally, a surface layer part of the steel is hardened by shot-peeningor the like, thereby applying residual stress to its outer surface. Incontrast, the seamless pipe can have its outer peripheral surfacesubjected to shot-peening in the same way, but its inner peripheralsurface cannot undergo the shot-peening. When decarburization occurs ata pipe surface layer located on the inner peripheral surface side of thepipe, adequate hardening on the inner peripheral surface side cannot beobtained during quenching in a spring production procedure, failing toensure fatigue strength required by springs. Furthermore, the presenceof a defect at the surface layer of the inner peripheral surface becomesa stress concentration part, which might cause the breakage of the pipeat an early stage.

During steel production, a small amount of hydrogen, which would causecracking, is inevitably introduced into and present in the steel. Such asmall amount of hydrogen is not problematic for the solid spring, butsignificantly affects the durability of a hollow spring. In particular,the hollow spring cannot have its inner surface subjected toshot-peening as mentioned above, and thus the hollow spring is requiredto have an even higher quality of resistance to hydrogen embrittlementthan the solid spring.

For these problems, some technical studies have taken place in terms ofproduction of a seamless pipe as a material. In a technique mentioned inPatent Document 1, hot isostatic pressing extrusion is performed on aworkpiece of steel to form a hollow seamless pipe shape, followed byspheroidizing annealing, and subsequently extending (drawing) the shapeby cold pilger mill rolling, cold drawing, or the like. As a result,according to a seamless steel tube of Patent Document 1, the depth ofcontinuous defects formed at the inner and outer peripheral surfaces ofthe steel tube can be reduced to 50 μm or less from each surface.

In a technique mentioned in Patent Document 2, a steel bar ishot-rolled, followed by perforation with a gun drill, and then issubjected to cold working (drawn, or rolled). As a result, a hollowseamless pipe for a high-strength spring of Patent Document 2 isproduced that can control a C content at the inner and outer peripheralsurfaces to 0.10% or more, while reducing the thickness of an entiredecarburized layer to 200 μm or less at each of the inner and outerperipheral surfaces.

Patent Document 3 has studied the relationship between the metalmicrostructure and durability of seamless pipes and thereby disclosing aseamless steel tube for a high-strength hollow spring in which a carbidehas a circle-equivalent diameter of 1.00 μm or less.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP 2007-125588 A-   Patent Document 2: JP 2010-265523 A-   Patent Document 3: JP 2011-184704 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

As a spring is strengthened, the resistance to hydrogen embrittlement ismore likely to be reduced. Thus, a spring is required to have excellentresistance to hydrogen embrittlement even with high strength.

The present invention has been made in view of the foregoingcircumstance, and it is a main object of the present invention toprovide a method for manufacturing steel for a high-strength hollowspring that exhibits excellent resistance to hydrogen embrittlement.Furthermore, it is another object of the present invention to provide amethod for manufacturing steel for a high-strength hollow spring thatexhibits excellent fatigue resistance.

Means for Solving the Problems

The method for manufacturing steel for a hollow spring according to thepresent invention that can solve the above-mentioned problems lies in amethod for manufacturing steel for a hollow spring obtained by quenchingand tempering a seamless pipe for use as a material of the hollowspring, a steel composition of the seamless pipe including, in percentby mass, C: 0.35 to 0.5%, Si: 1.5 to 2.2%, Mn: 0.1 to 1%, Cr: 0.1 to1.2%, Al: more than 0% and 0.1% or less, P: more than 0% and 0.02% orless, S: more than 0% and 0.02° or less, N: more than 0% and 0.02% orless, at least one element selected from the group consisting of V: morethan 0% and 0.2% or less, Ti: more than 0% and 0.2% or less, and Nb:more than 0% and 0.2% or less, and at least one element selected fromthe group consisting of Ni: more than 0% and 1% or less, and Cu: morethan 0% and 1% or less, wherein the quenching is performed to satisfyquenching conditions (1) mentioned below, and the tempering is performedto satisfy tempering conditions (2) mentioned below,

(1) quenching conditions:26000≤(T1+273)×(log(t1)+20)≤29,000900° C.≤T1≤1050° C.,10 seconds≤t1≤1,800 seconds,  (1)where T1 is a quenching temperature (° C.), and t1 is a holding time(seconds) in a temperature range of 900° C. or higher, and(2) Tempering Conditions:13,000≤(T2+273)×(log(t2)+20)≤15,500T2≤550° C., andt2≤3,600 seconds,  (2)where T2 is a tempering temperature (° C.), and t2 is a total time(seconds) from start of heating to completion of cooling.

The hydrogen content in the steel may be controlled to be 0 ppm or moreby mass and 0.16 ppm by mass or less.

Effects of the Invention

Effects obtained by the typical aspects of the present inventiondisclosed in the present application will be briefly described below.That is, the present invention constructed as mentioned above canmanufacture steel for a high-strength hollow spring that exhibitsexcellent resistance to hydrogen embrittlement even with high strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of a heat pattern takenwhen manufacturing steel for a hollow spring in the present invention.

MODE FOR CARRYING OUT THE INVENTION

The inventors have conducted various studies by using seamless pipes.Specifically, these studies have been executed in terms of optimizingrespective heat-treatment conditions for quenching and tempering to beperformed on the obtained seamless pipes, and not in terms of improvingthe quality of a seamless pipe as a material as mentioned in PatentDocuments 1 to 3. Consequently, it is found that when manufacturingsteel for a hollow spring by quenching and tempering a seamless pipethat has its steel composition appropriately controlled, the quenchingshould be performed to satisfy the quenching conditions (1) below, andthe tempering should be performed to satisfy the tempering conditions(2) below, where T1 is a quenching temperature (° C.); t1 is a holdingtime (seconds) in a temperature range of 900° C. or higher; T2 is atempering temperature (° C.), and t2 is a total time (seconds) fromstart of heating to completion of cooling, and thereby achieving thedesired objects of the present invention. Based on these findings, thepresent invention has been completed.

-   -   (1) quenching conditions:        26,000≤(T1+273)×(log(t1)+20)≤29,000        900° C. T1≤1,050° C.,        10 seconds≤t1≤1,800 seconds,  formula (1)        (2) Tempering Conditions:        13,000≤(T2+273)×(log(t2)+20)≤15,500        T2≤550° C., and        t2≤3,600 seconds.  formula (2)

Each of the terms “quenching temperature T1” and “tempering temperatureT2” as used herein means the surface temperature of a workpiece.Furthermore, each of the terms “temperature range of 900° C. or higher”,“heating start temperature”, and “cooling completion temperature” alsomeans the surface temperature of the workpiece. The surface temperaturecan be measured, for example, by a radiation thermometer, or by placinga thermocouple on the surface.

The term “quenching temperature” as used herein means a heatingtemperature (surface temperature) when quenching and hardening aseamless pipe.

First, the quenching conditions and tempering conditions whichcharacterize the present invention will be described in detail belowwith reference to FIG. 1. Note that in FIG. 1, t2 shows a time between aheating start temperature of 200° C. and a cooling completiontemperature of 200° C., based on examples to be mentioned later.However, the present invention is not limited thereto.

(1) Quenching Conditions:

The quenching conditions in the present invention are very important,particularly, to ensure the excellent resistance to hydrogenembrittlement even with high strength. It is supposed that quenching isperformed under the quenching conditions specified by the presentinvention, thus accelerating the refinement of prior austenite grains,an increase in the area of prior austenite grain boundaries, and anincrease in the amount of residual austenite, leading to the improvementof the durability, including embrittlement susceptibility to defects orhydrogen.

In the present invention, as specified by the formula (1) mentionedabove, the quenching parameter of “(T1+273)×(log(t1)+20)” which isrepresented by the balance between the quenching temperature T1 and theholding time t1 (seconds) in a temperature range of 900° C. or higher asshown in FIG. 1, needs to satisfy the range of 26,000 or higher and29,000 or lower. The formula (1) mentioned above is derived from variousbasic experiments under the following philosophy.

The tendency to accelerate the refinement of prior austenite grains, anincrease in the area of prior austenite grain boundaries, and anincrease in the amount of residual austenite after the quenching ispreferable from the viewpoint of the resistance to hydrogenembrittlement. Meanwhile, during heating in the quenching, the tendencyto accelerate the solid solution of carbides and to suppress the ferritedecarburization is preferable from the viewpoint of the resistance tohydrogen embrittlement. These factors are affected by both T1 and t1mentioned above, and hence it is necessary to appropriately control thebalance between T1 and t1. When taking into account the formerrequirements (the refinement of prior austenite grains, an increase inthe area of prior austenite grain boundaries, and an increase in theamount of residual austenite), the quenching at a low temperature for ashort period of time is considered to be preferable. On the other hand,from the viewpoint of accelerating the solid solution of carbides amongthe latter requirements (promotion of the solid solution of carbides andsuppression of the ferrite decarburization), the quenching at a hightemperature for a long period of time is considered to be preferable.Meanwhile, from the viewpoint of suppressing the ferritedecarburization, the quenching at a high temperature for a short periodof time is considered to be preferable. Considering thesecomprehensively, the above-mentioned formula (1) is specified.

In the formula (1), the upper limit of the quenching parameter ispreferably 28,700 or less, more preferably 28,500 or less, and stillmore preferably 28,300 or less. On the other hand, the lower limit ofthe quenching parameter is preferably 26,300 or more, and morepreferably 26,500 or more.

In the present invention, the quenching needs to be performed to satisfythe formula (1) as well as the following ranges: 900° C.≤T1≤1,050° C.and 10 seconds≤t1≤1,800 seconds. That is, among the values T1 and t1that can satisfy the range of the formula (1), the range of T1 and theupper limit of t1 are further limited to perform the quenching, therebyproducing the desired steel for a high-strength hollow spring.

The lower limit of the quenching temperature T1 is 900° C. or higher.This value is set from the following viewpoint. The quenchingtemperature needs to be set to at least the A₃ point or higher; the A₃point is a transformation temperature at which α (ferrite) istransformed into γ (austenite). In the component system of the presentinvention, the A₃ point is positioned at around 850° C. Note that interms of accelerating the solid solution of the carbides as mentionedabove, the quenching temperature should be higher. For this reason, thequenching temperature is set at the A₃ point+approximately 50° C. inmany cases. Under such a thought, also in the present invention, thelower limit of the quenching temperature T1 is set at 900° C., which isdetermined by formula below: 850° C. (A₃)+50° C.=900° C. From theviewpoint of accelerating the solid solution of carbides and furthersuppressing the ferrite decarburization, the T1 is preferably 920° C. orhigher, more preferably 925° C. or higher, and still more preferably930° C. or higher. Meanwhile, even if the upper limit of the T1 is sethigh, there is no problem as long as the processing time is short.However, T1 should not be extremely high when taking into account therefinement of the prior austenite grains, the increase in the area ofthe prior austenite grain boundaries, and the increase in the amount ofresidual austenite. Accordingly, in the present invention, the upperlimit of T1 is set at 1,050° C. or lower, preferably 1,020° C. or lower,and more preferably 1,000° C. or lower, and still more preferably 970°C. or lower.

The upper limit of the holding time t1 in the temperature range of 900°C. or higher is set at 1,800 seconds or less. The holding time t1 canalso be said to be a duration in which the temperature of the workpieceis passes through a temperature range of 900° C. or higher. If thequenching is performed while controlling the T1 in the range of 900° C.or higher, the solid solution of carbides can progress even for arelatively short period of time. However, when taking into account therefinement of the prior austenite grains, the increase in the area ofthe prior austenite grain boundaries, and the increase in the amount ofresidual austenite, t1 should not be so long. Accordingly, the t1 ispreferably 600 seconds or less, more preferably 300 seconds or less, andstill more preferably 100 seconds or less. Note that although the lowerlimit of the t1 can be set within the range that satisfies both theformula (1) and the above-mentioned range of T1, the lower limit of t1is 10 seconds or more when taking into account the actual operationallevel.

Here, the heat pattern in the above-mentioned “temperature range of 900°C. or higher” is not specifically limited as long as the quenchingconditions (1) are satisfied. For example, suppose that as shown in FIG.1, a heat pattern includes heating from 900° C. to T1 and then coolingfrom T1 to 900° C. The heating step may be performed at a certainaverage rate of temperature rise (e.g., 0.1 to 300° C./sec) such thatthe holding time t1 in a temperature range of 900° C. or highersatisfies the formula (1). The cooling step may be performed at acertain average rate of cooling (e.g., 0.1 to 300° C./sec). Asillustrated in FIG. 1, the heat pattern may include an isothermalholding step of holding a constant temperature within a temperaturerange of 900° C. or higher for a certain period of time. For example, anisothermal holding step to hold a temperature in a range of 900 to1,000° C. for 10 to 500 seconds may be included. These are examples ofthe pattern to which the present invention can be applied. In short, aslong as the quenching conditions (1) are satisfied, various heatpatterns can be adopted.

Furthermore, a heat pattern up to 900° C. is not also limitedspecifically. For example, as shown in FIG. 1, heating may be carriedout from room temperature to 900° C. (further to T1) at the same averagerate of temperature rise as that mentioned above. Alternatively, withinthe above-mentioned range of the average rate of temperature rise, theaverage rate of temperature rise may be set different depending on thetemperature range, for instance, a temperature range from the roomtemperature to 900° C. and a temperature range from 900° C. to T1.

After heating in the way mentioned above, rapid cooling (or quenching)is performed. For example, cooling is preferably performed from 900 to300° C. at an average cooling rate of approximately 20 to 1,000° C./sec.

(2) Tempering Conditions:

After quenching under the quenching conditions (1), tempering isperformed. The tempering conditions specified by the present inventionare very important, especially, in terms of ensuring excellent fatigueresistance. The tempering conditions specified by the present inventionare used, thereby increasing both the strength of the hollow spring andthe amount of residual austenite therein as well as appropriatelycontrolling the size and existence form of tempered carbides. As aresult, the durability, such as fatigue strength, of the hollow springis supposed to improve.

In the present invention, as specified by the above-mentioned formula(2), the tempering parameter of “(T2+273)×(log(t2)+20)” which isrepresented by the balance between the tempering temperature T2 (° C.)and the total time t2 (seconds) from start of heating to completion ofcooling as shown in FIG. 1, needs to satisfy the range of 13,000 or moreand 15,500 or less. The above-mentioned formula (2) is derived fromvarious basic experiments under the following philosophy.

In short, the term “total time t2 from the start of heating to thecompletion of cooling” as used herein means a total time spent by thetempering process. Specifically, this means the total period of timethat is taken to heat from the “heating start” temperature (e.g., in arange of the room temperature to 200° C.) to the tempering temperatureT2, and then to cool down to the “cooling completion” temperature (e.g.in a range of 200° C. to the room temperature). The reason why thepresent invention specifies the total time t2 spent by the temperingprocess as mentioned above rather a tempering time at the temperingtemperature T2 is that the tempering behavior progresses by heating.Note that as long as the above-mentioned requirements are satisfied, atempering holding time at the tempering temperature T2 is notparticularly limited. The “cooling completion temperature” in thepresent invention is 200° C. That is, the “cooling completion” isdefined as a state in which the surface temperature reaches 200° C. bycooling after heating up to the tempering temperature T2.

From the viewpoint of improving the strength and fatigue resistance, thetempering is preferably performed at a low temperature for a shortperiod of time. Note that as the strength of the hollow spring becomeshigh, the seamless pipe tends to have its resistance to hydrogenembrittlement degraded. For this reason, considering thesecomprehensively, the upper limit and lower limit of the above-mentionedformula (2) are specified in order to exhibit the excellent fatigueresistance.

In the formula (2), the upper limit of the tempering parameter ispreferably 15,200 or less, more preferably 15,000 or less, and stillmore preferably 14,700 or less. On the other hand, the lower limit ofthe tempering parameter is preferably 13,200 or more, more preferably13,500 or more, and still more preferably 13,700 or more.

The upper limit of t2 is 3,600 seconds or less when taking into accountthe actual operational level. The upper limit of t2 is preferably 2,400seconds or less. Note that the lower limit of t2 is not particularlylimited as long as it satisfies the tempering conditions represented bythe formula (2). However, when taking into account the actualoperational level, the lower limit of t2 is preferably approximately 10seconds or more.

The upper limit of T2 is 550° C. or lower. This is because as T2 isincreased, the fatigue resistance or the like is degraded. The upperlimit of T2 is preferably 500° C. or lower, and more preferably 450° C.or lower. The lower limit of T2 can be set to satisfy the rangerepresented by the formula (2). However, when taking into considerationa decrease in the strength of the hollow spring, the lower limit of T2is preferably 300° C. or higher, more preferably 325° C. or higher, andstill more preferably 350° C. or higher.

The heat pattern on the tempering conditions in the present invention isnot particularly limited as long as the above-mentioned requirements aresatisfied. For example, suppose that a heat pattern includes heatingfrom the room temperature to T2 and then cooling from T2 to the roomtemperature. An average rate of temperature rise in the heating step ispreferably controlled to be, for example, in a range of 1 to 300°C./sec. The average cooling rate in the cooling step is preferablycontrolled to be, for example, in a range of 1 to 1,000° C./sec. Asillustrated in FIG. 1, apart of the heat pattern may include anisothermal holding step of holding a constant temperature for a certainperiod of time. For example, an isothermal holding step to hold theconstant temperature as the T2 for 0 to 2,000 seconds may be included.When T2 is in a range of 200 to 450° C., T2 is preferably held at aconstant temperature for 10 to 2,000 seconds. These are examples of thepattern to which the present invention can be applied. In short, as longas the tempering conditions (2) are satisfied, various heat patterns canbe adopted.

The quenching conditions and tempering conditions featuring the presentinvention have been described above in detail.

The composition of the steel in the seamless pipe used as the materialwill be described. The composition of the steel in the seamless pipe inthe present invention is within a range normally used for a hollowspring. The reason for limiting the chemical components will bedescribed below.

[C: 0.35 to 0.5%]

Carbon (C) is an element required to ensure the strength of the steel.The lower limit of the C content is set at 0.35% or more. Thus, thelower limit of the C content is preferably 0.37% or more, and morepreferably 0.40% or more. However, any excessive C content degrades theductility of the steel. Thus, the upper limit of the C content is set at0.5% or less. The upper limit of the C content is preferably 0.48% orless, and more preferably 0.47% or less.

[Si: 1.5 to 2.2%]

Silicon (Si) is an element effective in exhibiting the fatigueresistance required for springs. To ensure setting resistance requiredfor a high-strength spring, the lower limit of the Si content is set at1.5% or more. The lower limit of the Si content is preferably 1.6% ormore, and more preferably 1.7% or more. However, Si is an element thataccelerates decarburization. Any excessive Si content disadvantageouslyaccelerates the formation of a decarburized layer on a steel surface.Thus, the upper limit of the Si content is set at 2.2% or less. Theupper limit of the Si content is preferably 2.1% or less, and morepreferably 2.0% or less.

[Mn: 0.1 to 1%]

Manganese (Mn) is used as a deoxidizing element while having effect torender harmful element sulfur (S) harmless by binding with S to formMnS. To effectively exhibit these effects, the lower limit of Mn contentis set at 0.1% or more. The lower limit of the Mn content is preferably0.15% or more, and more preferably 0.2% or more. However, any excessiveMn content forms segregation zones in the steel, which leads tovariations in the quality of material. Thus, the upper limit of the Mncontent is set at 1% or less. The upper limit of the Mn content ispreferably 0.9% or less, and more preferably 0.8% or less.

[Cr: 0.1 to 1.2%]

Chromium (Cr) is an element effective in ensuring the strength of steelafter the tempering and improving the corrosion resistance of steel.Thus, Cr is very important, particularly, for suspension springs thatare required to demonstrate the high-level corrosion resistance. Toeffectively exhibit these effects, the lower limit of the Cr content isset at 0.1% or more. The lower limit of the Cr content is preferably0.15% or more, and more preferably 0.2% or more. However, any excessiveCr content tends to easily generate a supercooled tissue and causeenrichment of Cr in cementite, reducing the plastic deformability of thesteel, thus leading to degradation in the cold forgeability thereof.Furthermore, any excessive Cr content tends to easily form Cr carbidesthat are different from cementite, thus worsening the balance betweenthe strength and ductility. Thus, the upper limit of Cr content is setat 1.2% or less. The upper limit of the Cr content is preferably 1.1% orless, and more preferably 1.0% or less.

[Al: More than 0% and 0.1% or Less]

Aluminum (Al) is added mainly as a deoxidizing element. Al binds with Nto form AlN, thereby rendering solid-solution N harmless, whilecontributing to refining the microstructure of the steel. To effectivelyexhibit these effects, the lower limit of the Al content is preferablyset at 0.005% or more, and more preferably 0.01% or more. However, sinceAl is a decarburization accelerating element, like Si, if the Si contentis large, the addition of an abundance of Al needs to be avoided. Thus,the upper limit of the Al content is set at 0.1% or less. The upperlimit of the Al content is preferably 0.07% or less, and more preferably0.05% or less.

[P: More than 0% and 0.02% or Less]

Phosphorus (P) is a harmful element that degrades the toughness andductility of the steel. For this reason, it is very important to reducethe P content. Thus, the upper limit of the P content is set at 0.02% orless. The upper limit of the P content is preferably 0.017% or less, andmore preferably 0.015% or less. Note that P is an impurity inevitablycontained in the steel, and hence the P content is difficult to set at0% in terms of industrial production.

[S: More than 0% and 0.02% or Less]

Like P mentioned above, sulfur (S) is a harmful element that degradesthe toughness and ductility of the steel. For this reason, it is veryimportant to reduce the S content. Thus, the upper limit of the Scontent is set at 0.02% or less. The upper limit of the S content ispreferably 0.017% or less, and more preferably 0.015% or less. Note thatS is an impurity inevitably contained in the steel, and hence the Scontent is difficult to set at 0% in terms of industrial production.

[N: More than 0% and 0.02% or Less]

Nitrogen (N) has an effect of refining the microstructure of the steelby forming a nitride in the presence of Al, Ti, and the like. Toeffectively exhibit this effect, the lower limit of the N content ispreferably set at 0.001% or more, and more preferably 0.002% or more.Note that the presence of N in a solid-solution state degrades thetoughness, ductility, and resistance to hydrogen embrittlement of thesteel. Therefore, the upper limit of N content is set at 0.02°. Theupper limit of the N content is preferably 0.01% or less, and morepreferably 0.007% or less.

[At Least One Element Selected from the Group Consisting of V: More than0% and 0.2% or Less, Ti: More than 0% and 0.2% or Less, and Nb: Morethan 0% and 0.2% or Less]

Vanadium (V), Titanium (Ti), and Niobium (Nb) bind with C, N, S, etc. toform precipitates, such as carbides, nitrides, carbonitrides, andsulfides, thereby rendering these elements harmless, such as C, N, andS. Such formation of the precipitates also exhibits the effect ofrefining an austenite microstructure during heating in an annealing stepof a manufacturing procedure for a seamless pipe, or in a quenching stepof a manufacturing procedure for a spring. Furthermore, these elementsalso have the effect of improving the delayed fracture resistance of thesteel. These elements may be used alone or in combination. Toeffectively exhibit these effects, the lower limit of the content of atleast one of Ti, V, and Nb (which means the content of a single elementwhen only one of them is included, or the total content of two or moreelements when two or more of them are included, and note that the samegoes for the following cases) is preferably 0.01% or more. However, anyexcessive content of the above-mentioned element(s) forms coarsecarbides, nitride, etc., leading to degradation in the toughness andductility of the steel in some cases. The upper limit of the content ofthe above-mentioned element(s) is set at 0.2% or less. The upper limitof the above-mentioned element(s) is preferably 0.18% or less, and morepreferably 0.15% or less.

[At Least One Element Selected from the Group Consisting of Ni: Morethan 0% and 1% or Less, and Cu: More than 0% and 1% or Less]

Nickel (Ni) and copper (Cu) are elements that are effective insuppressing the decarburization of a surface layer and improving thecorrosion resistance of the steel. These elements may be used alone orin combination.

Among them, Ni may not need to be added when taking into account thecost reduction. Thus, the lower limit of the Ni content is notparticularly limited. To effectively exhibit the above-mentioned effectby the addition of Ni, the lower limit of the N content is preferablyset at 0.2% or more. Note that any excessive Ni content generate asupercooled tissue in a rolled material and leaves residual austeniteafter the quenching, thereby degrading the fatigue resistance and thelike in some cases. Thus, the upper limit of the Ni content is set at 1%or less. Further, when taking into consideration the cost reduction andthe like, the upper limit of the Ni content is preferably 0.8% or less,and more preferably 0.6% or less.

To effectively exhibit the above-mentioned effect by the addition of Cu,the lower limit of the C content is preferably set at 0.2% or more. Notethat like Ni, any excessive Cu content generates the supercooled tissue,causing cracks during hot working in some cases. Thus, the upper limitof the Cu content is set at 1% or less. Further, when taking intoconsideration the cost reduction, the upper limit of the Cu content ispreferably 0.8% or less, and more preferably 0.6% or less.

The basic components of the seamless pipe used in the present inventionhave been mentioned above, with the balance being iron and inevitableimpurities. Examples of the inevitable impurities can include Sn and As.The smaller the content of the inevitable impurity, the better the steelof the seamless pipe normally becomes, for example, like P and S. Forthis reason, particularly, even some inevitable impurities have theupper limits of their contents additionally specified as mentionedabove. Thus, the term “inevitable impurity” as used herein, whichconfigures the balance, is defined as another element other than theelement, an upper limit of whose content is specified as mentioned abovein terms of concept.

The method for manufacturing steel for a hollow spring according to thepresent invention involves performing (1) quenching and (2) tempering ona seamless pipe with a predetermined composition, as mentioned above.Other steps are not particularly limited, and a normal method can beadopted therefor. Now, a description will be given on the preferablemethod for manufacturing steel for a hollow spring.

First, steel with the predetermined composition is smelted by a normalsmelting method, followed by cooling (i.e., casting) an obtained moltensteel.

Thereafter, blooming is performed on the steel. The heating temperaturefor the blooming is preferably in a range of, for example, 1,100 to1,300° C.

Then, a slab obtained by the above-mentioned blooming is subjected tohot forging to be formed into a round bar. The heating temperature forthe hot forging is preferably in a range of, for example, 1,000 to1,200° C.

Thereafter, the seamless pipe may be produced by the known method. Forinstance, after the hot forging, the round bar is formed into apredetermined shape by using the known piercing method, followed by hotextrusion, cooling, cold working, annealing, pickling, and if necessary,polishing of an inner surface layer and cold working, thereby producinga seamless pipe.

Among the above-mentioned steps, the annealing after the cold working ispreferably performed by heating up to a temperature range of A₃ point orhigher and 1,000° C. or lower. The holding time in the temperature rangeof A₃ point or higher, that is, the total time after the start ofheating at the temperature of A₃ point or higher until when thetemperature of A₃ point is reached by cooling is preferably controlledto be five minutes or less. In this way, the holding time is controlledwithin the above-mentioned range, so that the occurrence ofdecarburization during annealing and the like is suppressed, andcarbides are refined, thereby making it possible to improve the fatigueproperties.

Here, the A₃ point can be determined as follows. Note that [ ] in theformula below indicates % by mass. For example, [C] means the C contentin % by mass.A₃=894.5−269.4×[C]+37.4×[Si]−31.6×[Mn]−19.0×[Cu]−29.2×[Ni]−11.9×[Cr]+19.5×[Mo]+22.2×[Nb]

The annealing after the above-mentioned cold working is preferablyperformed in an inert or reducing gas atmosphere. Such control of theannealing atmosphere can suppress the occurrence of decarburization inannealing. Furthermore, the generation of scales during annealing can besuppressed, which can omit a pickling step.

The pickling time in manufacturing the seamless pipe is preferablycontrolled to be 30 minutes or less, or alternatively the picklingitself is preferably omitted. In this way, the hydrogen content in theseamless pipe can be reduced, whereby the hydrogen content after thetempering and quenching can also be reduced.

After producing the seamless pipe in the way above, in a springformation procedure, such as hot forming or cold forming, the quenchingprocess and tempering process are performed to obtain the steel for ahollow spring. In the case of the hot forming, after producing theseamless pipe, the quenching under the conditions (1) is performed. Atthis time, during heating for the quenching, spring forming is alsoperformed, and then the tempering is performed under the conditions (2).On the other hand, in the case of the cold forming, after producing theseamless pipe, the quenching under the conditions (1) and the temperingunder the conditions (2) are performed, and then spring forming isperformed without heating.

Furthermore, the hydrogen content in the steel for a hollow springobtained by the manufacturing method according to the present inventionis preferably controlled to be 0 ppm by mass or more and 0.16 ppm bymass or less.

Since shot-peening cannot be applied to the inner peripheral surface ofthe hollow spring as mentioned above, there are strict requirements forthe durability of hollow springs, regarding the embrittlementsusceptibility to defects or hydrogen. Even a small amount of hydrogenin the steel for a hollow spring significantly affects the durability ofthe spring. Thus, the upper limit of the hydrogen content is preferably0.16 ppm or less by mass. Consequently, as shown in Examples to bementioned later, the very high fatigue resistance can be achieved.Therefore, the smaller the hydrogen content, the better the quality ofthe steel for a hollow spring becomes. The upper limit of theabove-mentioned hydrogen content is preferably 0.15 ppm or less by mass,and more preferably 0.14 ppm or less by mass.

A method for reducing the hydrogen content in the steel for a hollowspring is well known. Even in the present invention, the methodconventionally used can be selected and applied as appropriate. In aspecific example of the reducing method of the hydrogen content in thesteel, for example, a pickling time in a seamless pipe production stepis shorten to approximately 30 minutes or less. Alternatively, picklingitself may be omitted. Alternatively, a dehydrogenation process may beperformed after the quenching and tempering in manufacturing the steelfor a hollow spring. The dehydrogenation process can be performed, forexample, by applying heat treatment at 300° C. or lower.

The method for manufacturing steel for a hollow spring according to thepresent invention has been described above.

The steel for a hollow spring obtained in this way is used and finallysubjected to processes, including setting and shot-peening, therebyproducing a hollow spring. Note that when performing the cold forming asmentioned above, the spring forming may be performed on the steel for aspring, and then setting and shot-peening may be performed thereon.

Examples of the hollow spring include a valve spring, a clutch spring,and a suspension spring. The hollow spring is suitable for use in theengines, clutches, suspensions of automobiles, and the like.

EXAMPLES

The present invention will be more specifically described below by wayof Examples, but is not limited to the following Examples. Variousmodifications and changes can be made to these examples as long as theyare adaptable to the above-mentioned and below-mentioned concepts, andthey are included within the technical scope of the present invention.

As mentioned above, the most characteristic aspect of the presentinvention is that a predetermined heat treatment is applied to aseamless pipe. The inner peripheral surface or outer peripheral surfaceof the seamless pipe subjected to the heat treatment has substantiallythe same surface texture as an outer peripheral surface of a solid steelmaterial subjected to the heat treatment. Thus, the presence or absenceof the effects of the present invention is not linked to the shape ofthe material. Therefore, in Examples 1 and 2 mentioned below, not theseamless pipe, but the solid steel material was used. Respective heattreatments of the quenching and tempering specified by the presentinvention were applied to the steel material, which was then evaluated.

Example 1

In this example, to clarify the influences of the quenching andtempering conditions, especially, on the hydrogen embrittlementsusceptibility, experiments were conducted in the following way. Here, asteel No. Al shown in Table 1, which was a medium carbon steelsatisfying the requirement of the present invention, was used.

First, after smelting the steel by a normal smelting method, theobtained molten steel was cooled (i.e., casted), and then subjected toblooming by heating to 1,100 to 1,300° C., thereby producing a slab witha cross-sectional shape of 155 mm×155 mm. Then, the hot forging wasperformed on the slab on a heating condition, namely, at 1,000 to 1,200°C., thereby forming a round bar with a diameter of 150 mm. Then, the hotforging was further performed by heating on a heating condition, namely,at 1,000 to 1,200° C., thereby producing a round bar with a diameter of15 mm.

TABLE 1 Steel Chemical composition* (% by mass) type C Si Mn Cr Al P S NV Ti Ni Cu A1 0.43 1.90 0.21 0.95 0.0350 0.007 0.007 0.0040 0.145 0.0800.60 0.31 *Balance: Iron and inevitable impurities other than P and S

The round bars obtained in this way were subjected to various quenchingand tempering processes shown in Table 2, thereby cutting outflat-shaped specimens, each having 10 mm width×1.5 mm thickness×65 mmlength. Each flat-shaped specimen was used and evaluated for theresistance to hydrogen embrittlement and Vickers hardness in thefollowing way.

In detail, the conditions for the quenching and tempering were asfollows. The steel round bar was heated at an average rate oftemperature rise of 10° C./sec in a temperature range from the roomtemperature to T1, and then held at T1 for a predetermined time. Then,the steel bar was cooled at an average cooling rate of 50° C./sec in atemperature range from T1 to 300° C. At this time, the holding time atT1 was changed such that the holding time t1 at 900° C. or higher was600 seconds.

Subsequently, the steel bar was cooled down to 200° C., and thensubjected to the tempering. Specifically, the steel bar was heated at anaverage rate of temperature rise of 10° C./sec in a temperature rangefrom 200° C. to T2, and then held at T2 for a predetermined time. Then,the steel bar was cooled at an average cooling rate of 300° C./sec in atemperature range from T2 to 200° C. At this time, the holding time atT2 was changed such that t2 (the time after heating to 200° C. or higherbefore cooling to 200° C. or lower) was 2,400 seconds.

(Evaluation on Resistance to Hydrogen Embrittlement)

Each specimen, which was obtained as mentioned above, with a stress of1,400 MPa applied thereto by four point bending was immersed in 1 L of amixed solution that contained 0.5 mol of sulfuric acid and 0.01 mol ofpotassium thiocyanate. A voltage of −700 mV, which was lower than asaturated calomel electrode (SCE), was applied to the specimen by usinga potentiostat, and a time (fracture time) until a crack occurred wasmeasured. In this example, specimens having a fracture lifetime of 1,000seconds or more were rated as “pass”.

(Vickers Hardness)

The plate-shaped specimen was embedded in resin such that itscross-section in the width-thickness direction was exposed, followed bypolishing and mirror-finish. Then, a Vickers hardness (Hv) of thespecimen was measured by applying a load of 500 g to the positionlocated at the center in the depth direction from the surface layer ofthe specimen. In this example, specimens having a Vickers hardness of550 Hv or higher were rated as having a high strength. These results ofthe evaluation are shown together in Table 2.

TABLE 2 Resistance to hydrogen Quenching conditions (1) Temperingconditions (2) embrittlement Strength Temperature Temperature FractureVickers Specimen T1 Time t1 Quenching T2 Time t2 Tempering lifetimehardness No. (° C.) (seconds) parameter (° C.) (seconds) parameter(seconds) (Hv) 1 900 600 26,719 300 2,400 13,397 1,186 627.0 2 900 60026,719 325 2,400 13,981 1,659 621.8 3 900 600 26,719 350 2,400 14,5661,300 616.5 4 900 600 26,719 375 2,400 15,150 1,375 611.3 5 900 60026,719 400 2,400 15,735 990 582.0 6 900 600 26,719 425 2,400 16,3191,372 540.5 7 900 600 26,719 450 2,400 16,904 1,337 506.0 8 925 60027,288 300 2,400 13,397 1,800 625.3 9 925 600 27,288 325 2,400 13,9811,390 620.0 10 925 600 27,288 350 2,400 14,566 1,799 618.3 11 925 60027,288 375 2,400 15,150 1,609 599.0 12 925 600 27,288 400 2,400 15,735888 582.0 13 925 600 27,288 425 2,400 16,319 1,501 533.5 14 925 60027,288 450 2,400 16,904 1,465 507.3 15 1,025 600 29,566 300 2,400 13,397914 614.8 16 1,025 600 29,566 325 2,400 13,981 980 607.8 17 1,025 60029,566 350 2,400 14,566 918 609.5 18 1,025 600 29,566 375 2,400 13,150880 599.0 19 1,025 600 29,566 400 2,400 15,735 350 583.8 20 1,025 60029,566 425 2,400 16,319 570 533.3 21 1,025 600 29,566 450 2,400 16,9041,297 509.8

Specimen Nos. 1 to 4 and 8 to 11 shown in Table 2 are examples in whichthe steels satisfying the requirements of the present invention wereused to perform the quenching (1) and tempering (2) specified by thepresent invention. All these specimens had a long fracture lifetime of1,000 seconds or more, though they had high strength. Thus, suchspecimens had excellent resistance to hydrogen embrittlement.

In contrast, the specimen Nos. 5 to 7 are examples in which the samequenching conditions were used and their respective tempering parametersexceeded the upper limit of the tempering parameter specified by theformula (2). The numerical value of the tempering parameter wasincreased from the specimen No. 5 to the specimen Nos. 6 and No. 7 inthis order. The specimen No. 5 that had its tempering parameter slightlyexceeding the upper limit thereof had adequate hardness, but a shortfracture lifetime. On the other hand, in each of the specimen Nos. 6 and7, as the numerical value of the tempering parameter was increased, thehardness of the steel was reduced, but the fracture lifetime was notless than 1,000 seconds, which was specified by the present invention.

The same tendency as those observed in the specimen Nos. 5 to No. 7 werealso recognized in specimen Nos. 12 to 14. That is, the specimen Nos. 12to 14 are other examples in which the same quenching conditions wereused and their respective tempering parameter exceeded the upper limitof the tempering parameter specified by the formula (2). The numericalvalue of the tempering parameter was increased from the specimen No. 12to the specimen No. 13 and the specimen No. 14 in this order. Thespecimen No. 12 that had its tempering parameter slightly exceeding theupper limit thereof had adequate hardness, but a short fracturelifetime. On the other hand, in each of the specimen Nos. 12 and 13, asthe numerical value of the tempering parameter was increased, thehardness of the steel was reduced, but the fracture lifetime was notless than 1,000 seconds which was specified by the present invention.

As can be seen from these results, the upper limit of temperingparameter was found to be a very important factor that ensures thedesired high strength and the properties of the resistance to hydrogenembrittlement. Therefore, it was confirmed that only by controlling theupper limit of the tempering parameter within the range specified by thepresent invention, the above-mentioned desired properties wereexhibited.

The specimen Nos. 15 to 21 are examples in which the same quenchingconditions were used and their respective tempering parameters slightlyexceeded the upper limit of the quenching parameter specified by theformula (1).

Among the specimens mentioned above, the specimen Nos. 15 to 18 areexamples in which the tempering conditions (2) specified by the presentinvent ion were used in the manufacturing procedure. However, thequenching parameter of each of these specimens exceeded the upper limitthereof, resulting in a short fracture lifetime.

On the other hand, the specimen Nos. 19 to 21 are examples in whichtheir tempering parameters exceeded the upper limit of the temperingparameter specified by the formula (2). The numerical value of thetempering parameter was increased from the specimen No. 19 to thespecimen Nos. 20 and No. 21 in this order. The specimen No. 19 that hadits tempering parameter slightly exceeding the upper limit thereof hadadequate hardness, but a short fracture lifetime. On the other hand, ineach of the specimen Nos. 20 and 21, as the numerical value of thetempering parameter was increased, the hardness of the steel wasreduced, but the fracture lifetime was increased. In the specimen No.21, the fracture lifetime was not less than 1,000 seconds specified bythe present invention, and the resistance to hydrogen embrittlement wasimproved.

As can be seen from these results, the upper limit of quenchingparameter was found to be a very important factor that ensures thedesired resistance to hydrogen embrittlement. Therefore, it wasconfirmed that if the upper limit of the quenching parameter does notsatisfy the range of the present invention, the desired propertiescannot be obtained.

Example 2

In this example, particularly, to clarify the influences of thequenching and tempering conditions on the fatigue resistance,experiments were conducted using the round bar produced in Example 1 inthe following way.

(Evaluation on Fatigue Resistance)

After performing the quenching and tempering on the round bars undervarious conditions mentioned in Table 3, each round bar was processed toproduce a specimen in conformity with JIS standard (a specimen for afatigue test in accordance with JIS Z2274). Then, the rotational bendingfatigue test was performed on the specimen at a rotational speed of 3000rpm with a stress of 900 MPa applied thereto. The details of thequenching and tempering conditions were the same as those mentioned inExample 1. In this example, specimens in which the number of cycles thatcaused failure was 100,000 or more were rated as “pass”.

These results of the evaluation are shown together in Table 3. Thespecimen Nos. 10 and 17 shown in Table 3 corresponded to the specimenNos. 10 and 17 shown in Table 2, respectively. Further, the specimenNos. 10 and 17 in Table 3 had the same heat treatment conditions as thespecimen Nos. 10 and No. 17 in Table 2, respectively.

TABLE 3 Fatigue resistance Quenching conditions (1) Tempering conditions(2) Number of Temperature Temperature cycles to Specimen T1 Time t1Quenching T2 Time t2 Tempering failure No. (° C.) (seconds) parameter (°C.) (seconds) parameter (cycles) 10 925 600 27,288 350 2,400 14,566161,500 22 925 600 27,288 430 2,400 16,436 62,100 17 1,025 600 29,566350 2,400 14,566 594,400 23 1,025 600 29,566 430 2,400 16,436 62,100

First, the specimen No. 10 will be compared with the specimen No. 17.These specimens are examples in which the tempering was performed on thesame tempering conditions, which were specified by the presentinvention, but these specimens differ from each other in the quenchingconditions. The specimen No. 10 was the example that satisfied thequenching conditions specified by the present invention, while thespecimen No. 17 was the example in which its quenching parameterslightly exceeded the upper limit of the quenching parameter specifiedby the present invention.

As shown in Table 3, when focusing on only the fatigue resistance, adifference in the quenching condition did not lead to a differentevaluation result in terms of the fatigue resistance. Even if thequenching was performed with its parameter exceeding the upper limit ofthe quenching parameter, like the specimen No. 17, the adequate fatigueresistance was obtained in the same manner as when the quenchingconditions specified by the present invention were used, like thespecimen No. 10. Note that as shown in Table 2 mentioned above, in thespecimen No. 17, its tempering parameter exceeded the upper limit of thetempering parameter, thus decreasing the fracture lifetime. To satisfythe desired resistance to hydrogen embrittlement and high-strength, itis confirmed that the achievement of both the quenching condition andtempering condition specified by the present invention is essential.

Next, the specimen No. 22 will be compared with the specimen No. 23.These specimens are examples in which the tempering was performed on thesame tempering conditions, but their tempering parameters exceeded thetempering parameter specified by the present invention. Furthermore,these specimens differ from each other in the quenching conditions. Thespecimen No. 22 was the example that satisfied the quenching conditionsspecified by the present invention, while the specimen No. 23 was theexample in which its quenching parameter slightly exceeded the upperlimit of the quenching parameter specified by the present invention.

As shown in Table 3, both the specimen Nos. 22 and 23 deviated from thetempering conditions specified by the present invention, thus leading todegradation in the fatigue resistance. Thus, when focusing on only thefatigue resistance, a difference in the quenching condition did not leadto a different evaluation result in terms of a criterion of the fatigueresistance. For instance, even if the quenching was performed with itsparameter exceeding the upper limit of the quenching parameter, like thespecimen No. 23, the fatigue resistance was degraded in the same manneras when the quenching conditions specified by the present invention wereused, like the specimen No. 22.

Example 3

In this example, to clarify the influences of the tempering conditions,especially, on the fatigue resistance by using the steel for a hollowspring, seamless pipes were produced in the following way. Then, thehydrogen content in the steel of each seamless pipe was measured, andthe fatigue resistance of the steel was evaluated.

(Measurement of Hydrogen Content in Steel)

The round bar with a diameter of 150 mm produced in Example 1 mentionedabove was used and machined to produce an extrusion billet, followed byhot extrusion at 1,100° C. as a heating condition, thus producing anextrusion tube with an outer diameter of 54 mm and an inner diameter of37 mm. Then, after cold working (in detail, drawing process:non-continuous draw bench, rolling process: Pilger rolling mill),annealing was performed on the tube at a temperature of 920 to 1,000° C.for a total heating time of 20 minutes or less, the total heating timebeing measured at the temperature of 900° C. or higher. Subsequently, toadjust the hydrogen content in the steel for each tube, the pickling wasperformed by changing the pickling time for the corresponding tube.Specifically, the pickling process was performed by pickling the steeltube in a pickling solution of 5 to 10° hydrochloric acid for 10 to 30minutes. Then, the cycle of cold working, annealing, and pickling wasrepeated a plurality of times, thereby producing a seamless pipe with anouter diameter of 16 mm and an inner diameter of 8.0 mm.

The seamless pipe obtained in this way was subjected to the quenchingprocess and the tempering process. The detailed conditions for thequenching and tempering were as follows. First, the seamless pipe washeated at an average rate of temperature rise of 100° C./sec in atemperature range from the room temperature to T1, and then held at T1for a predetermined time. Then, the seamless pipe was cooled at anaverage cooling rate of 50° C./sec in a temperature range from T1 to300° C. At this time, the holding time at T1 was changed such that theholding time t1 at 900° C. or higher was 60 seconds.

Subsequently, after being cooled to 200° C., the seamless pipe wassubjected to the tempering. Specifically, the seamless pipe was heatedat an average rate of temperature rise of 10° C./sec in a temperaturerange from 200° C. to T2, and then held at T2 for a predetermined time.Subsequently, the seamless pipe was cooled at an average cooling rate of300° C./sec in a temperature range from T2 to 200° C. At this time, theholding time at T2 was changed such that t2 (the time after heating to200° C. or higher before cooling to 200° C. or lower) was 2,400 seconds.

In this way, a ring-shaped specimen with a width of 1 mm was cut out ofthe obtained steel for a hollow spring, and then the amount ofdischarged hydrogen from the specimen was measured. The amount ofdischarged hydrogen was measured through temperature elevation analysisby an atmospheric pressure ionization mass spectrometry (APIMS). Here,the rate of temperature rise was set at 720° C./hr, and the hydrogencontent in the steel was defined as the amount of discharged hydrogenuntil 720° C.

(Measurement of Fatigue Resistance)

The steel for a hollow spring of each specimen was used and evaluatedfor the fatigue resistance. In this example, a torsion fatigue test wasperformed on the steel at a load stress of 735±600 MPa. Specimens havingthe number of cycles to failure of 50,000 or more were rated as havingexcellent fatigue resistance.

These results of this evaluation are shown in Table 4.

TABLE 4 Fatigue resistance Quenching conditions (1) Tempering conditions(2) Hydrogen Number of Temperature Temperature content cycles toSpecimen T1 Time t1 Quenching T2 Time t2 Tempering in steel failure No.(° C.) (seconds) parameter (° C.) (seconds) parameter (ppm) (cycles) 11,020 60 28,159 350 2,400 14,566 0.16 297,000 2 1,020 60 28,159 3502,400 14,566 0.18 70,700 3 1,020 60 28,159 390 2,400 15,501 0.15 37,4004 1,020 60 28,159 390 2,400 15,501 0.26 30,200

In the specimen Nos. 1 to 4 shown in Table 4, all their quenchingconditions were the same, and the quenching was performed on theconditions specified by the present invention. However, the specimensdiffered from one another in the tempering conditions. The specimen Nos.1 and 2 are the examples in which the tempering conditions specified bythe present invention were used. The specimen Nos. 3 and 4 are theexamples in which their tempering parameters slightly exceeded the upperlimit of the tempering parameter specified by the present invention.

When comparing between the specimen Nos. 1 and No. 2, in the specimenNo. 1, a hydrogen content in the steel was controlled to be 0.16 ppm bymass, which was the preferable upper limit specified by the presentinvention, whereas in the specimen No. 2, a hydrogen content was notcontrolled to be the upper limit. Thus, the specimen No. 1 achieved thesignificantly large number of cycles to failure and exhibited theextremely high fatigue resistance, compared to the specimen No. 2.

In contrast, when the tempering was performed with its temperingparameter slightly exceeding by only 1 the upper limit thereof (15,500)specified by the present invention, like the specimen Nos. 3 and No. 4,the number of cycles to failure was decreased. Even if the hydrogencontent in the steel was controlled to be the preferable upper limit,like the specimen No. 3, the number of cycles to failure could not reach50,000, which was a criterion for “pass”.

As can be seen from these results, it was confirmed that to ensure thefatigue resistance of the hollow spring, it is very important toappropriately control, especially, the tempering conditions. Whencontrolling the upper limit of the hydrogen content in the steel withina preferable range, in addition to the tempering process on thetempering conditions specified by the present invention, it was foundthat the fatigue resistance was improved drastically.

In Example 3, the fracture lifetime serving as an index of theresistance to hydrogen embrittlement was not measured. However, sincethe specimen Nos. 1 and 2 satisfied the quenching conditions (1), it isconsidered that the specimen Nos. 1 and 2 achieved the adequateresistance to hydrogen embrittlement.

The present application claims priority to Japanese Patent ApplicationNo. 2014-222840, filed on Oct. 31, 2014, the disclosure of which isincorporated herein by reference in its entirety.

The invention claimed is:
 1. A method for manufacturing steel, themethod comprising: quenching and tempering a seamless pipe comprising asteel composition comprising, in percent by mass: C: 0.35 to 0.5%, Si:1.5 to 2.2%, Mn: 0.1 to 1%, Cr: 0.1 to 1.2%, Al: more than 0% and 0.1%or less, P: more than 0% and 0.02% or less, S: more than 0% and 0.02% orless, N: more than 0% and 0.02% or less, at least one element selectedfrom the group consisting of V: more than 0% and 0.2% or less, Ti: morethan 0% and 0.2% or less, and Nb: more than 0% and 0.2% or less, and atleast one element selected from the group consisting of Ni: more than 0%and 1% or less, and Cu: more than 0% and 1% or less, wherein thequenching is performed to satisfy quenching conditions (1), and thetempering is performed to satisfy tempering conditions (2), (1)quenching conditions:26,000≤(T1+273)×(log(t1)+20)≤29,000900° C.≤T1≤1,050° C., and10 seconds≤t1≤1,800 seconds,  formula (1) wherein T1 is a quenchingtemperature by ° C., t1 is a duration time in quenching by seconds,which is timed starting at a moment when the pipe reaches 900° C. andending at a moment when the pipe reaches 900° C. after the pipe is heldat the quenching temperature T1 for a quenching holding time, when thequenching temperature T1 is 900° C., the duration time t1 equals to thequenching holding time, and when the quenching temperature T1 is higherthan 900° C., the duration time t1 is greater than the quenching holdingtime; and (2) tempering conditions:13,000≤(T2+273)×(log(t2)+20)≤15,500  formula (2)T2≤550° C., andt2≤3,600 seconds, wherein T2 is a tempering temperature by ° C., and t2is a total time in tempering by seconds, which is timed starting at amoment when the pipe reaches a heating start temperature and ending at amoment when the pipe reaches a cooling completion temperature after thepipe is held at the tempering temperature T2 for a tempering holdingtime.
 2. The method according to claim 1, wherein the hydrogen contentin the steel is controlled to be 0 ppm or more by mass and 0.16 ppm bymass or less.
 3. The method according to claim 1, wherein the temperingconditions (2) are:13,000≤(T2+273)×(log(t2)+20)≤15,200T2≤550° C., andt2≤3,600 seconds.
 4. The method according to claim 1, wherein in thetempering, the tempering temperature T2 satisfies 300° C.≤T2≤550° C.,the heating start temperature ranges from room temperature to 200° C.,and the cooling completion temperature ranges from 200° C. to roomtemperature.